Energy & Fuels 2003, 17, 363-368
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CaS Oxidation by Reaction with CO2 and H2O E. J. Anthony,* L. Jia, and K. Qiu CETC, Natural Resources Canada, 1 Haanel Drive, Ottawa, Canada K1A 1M1 Received June 5, 2002
Calcium sulfide is an undesirable product from both FBC and topping cycle gasifiers. Unfortunately, its direct oxidation in a FBC environment is made difficult due to the fact that unreacted CaS is protected from further reaction with O2 by formation of a CaSO4 shell. In previous work it was often implied that oxidation of CaS occurred solely by reaction with O2. However, this study shows that CO2 can be an effective oxidant for CaS at temperatures above 600 °C. Furthermore, it appears that the principal gas-phase products are SO2 and CO, and direct attempts to measure COS using mass spectrometry suggest that its formation is negligible. Somewhat surprisingly, SO2/CO molar ratios are in the range of 0.4 to 0.5, which is problematic, if the dominant reaction is CaS + 3CO2 d CaO + 3CO + SO2, unless CO2 is reacting to form elemental carbon, which appears unlikely, or side reactions occur, leading to a variety of gaseous products. These results also suggest that CO itself has no significant part in the oxidation process at least up to 850 °C or more. Reaction with H2O also begins above 600 °C, but it is a much less effective oxidant, and experiments with CO2/H2O further support the idea that CO2 is the more important oxidant and might be used to destroy CaS from topping cycle gasifiers.
Introduction Topping cycle gasifiers burning high-sulfur fuels must employ some method of sulfur removal, typically by means of limestone or dolomite addition.1 The CaS so produced is then transferred together with unburnt char to another reactor, either a pressurized fluidized bed (PFBC) or circulating fluidized bed combustor (CFBC), where it is hoped that both char burnout and oxidation of CaS to CaSO4 will occur. Unfortunately, it has proven to be much more difficult to oxidize the CaS in a FBC environment than hitherto supposed and this has been discussed elsewhere.2 One solution is to use dolomites that have been shown to give good conversion of CaS.1 However, the use of dolomitic limestones in FBC has a number of problems, not the least of which is their lack of general availability, and the fact that the Mg component is unable to participate in sulfur capture at typical FBC/gasification conditions. Calcitic limestones are, therefore, due to these considerations, the preferred type of sorbent, although effective elimination of CaS may then not occur in a PFBC or CFBC environment.2 Oxidation by O2 is often assumed to be the sole effective route for CaS oxidation although Illerup and co-workers3,4 did carry out limited regeneration tests of sulfided limestone at 850, 900, and 950 °C with a CO2/ * Corresponding author. Fax: 613-992-9335. E-mail: banthony@ nrcan.gc.ca. (1) Sage, P. E.; Schofield, P. A.; Gaya´n, P. Minimization of Calcium Sulphide in Gasification Residues by Combustion in a Circulating Fluidized Bed. Proceedings of the 14th International Conference on FBC, Preto, F. D. S., Ed.; Vancouver, BC, May 11-14, 1997; p 847856, (2) Qiu, K.; Anthony, E. J.; Jia, L. Oxidation of sulfided limestone under the conditions of pressurized fluidized bed combustion. Fuel 2001, 80, 549-558. (3) Illerup, J. B. Hydrogen Sulfide and Sulfur Dioxide Retention on Limestone at High Temperature and High Pressure. Ph.D. Thesis, Technical University of Denmark, 1994.
N2 mixture. A reaction system of CaS-O2-CO2 has been studied by Qiu et al.2 Thus, for instance, when CaS was reacted with a mixture of 4% O2/80% CO2/16% N2 at 850 °C, it was found that the solid products were CaSO4 and CaCO3, with the conversion being 42% and 6% in 150 min, respectively. Regeneration of sulfided dolomite has also been studied using various mixtures of O2/CO2 and H2O.5,6 More recently, Qiu et al.2 have shown that CO2 can convert CaS, under both pressurized and atmospheric conditions over a wide range of temperatures, to CaO, CaSO4, and CaCO3. However, the exact mechanism by which these transformations occur is far from clear, although it is possible to write a series of balanced equations in which CaS is converted to these products, such as:
CaS + CO2 ) CaO + COS
(1)
CaS + 2CO2 ) CaSO4 + 2C
(2)
CaS + 3CO2 ) CaO + SO2 + 3CO
(3)
CaS + 4CO2 ) CaSO4 + 4CO
(4)
CaS + 3CaSO4 ) 4CaO + 4SO2
(5)
The choice between CaO and CaCO3 formation is determined by whether the CO2 partial pressure exceeds that for CaCO3 formation (for these experiments the (4) Illerup, J. B.; Dam-Johansen, K.; Johnsson, J. E. Hydrogen Sulfide Retention on Limestone at High Temperature and High Pressure. In Gas Cleaning at High Temperature; Clift, R., Seville, J. P. K., Eds.; Blackie Academic and Professional: London, 1993; pp 492509. (5) Turkdogan, E. T.; Viners, J. V. Desulphurization of Hot Reducing Gases with Calcined Dolomites. Ironmaking Steelmaking, 1978, 4, 168-176. (6) Chou, C. L.; Li, K. Kinetic and Structural Studies of Regeneration of Sulfided Dolomite in the Carbon Dioxide - II. The Cyclic Regeneration. Chem. Eng. Commun. 1984, 29, 181-200.
10.1021/ef020124s CCC: $25.00 © 2003 American Chemical Society Published on Web 01/31/2003
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limiting temperature for CaCO3 formation occurs at 898 °C). It has also been suggested in the work of Qiu et al.2 that CaSO4 formation might occur due to a secondary reaction between SO2 and CaO. An implication of the overall reactions is that on a molar basis, more CO than SO2 should be produced. If reactions 3 or 4 + 5 dominate, the SO2/CO molar ratio should be 1/3. With CaSO4 formation as in reactions 2 or 4, even less SO2 would be formed. It should be noted that reaction 5 does not occur below about 850-900 °C.7-9 Illerup et al.3 have analyzed the thermodynamic equilibrium for a reaction system of CaS-85% CO2/14% N2/1% H2O at a pressure of 1 bar. Their calculations show that at 1100 °C a SO2 concentration of 2% is achieved, which is sufficient to allow it to be collected to make sulfur byproducts. It was decided to examine the reaction of CaS in a CO2 gas stream using CO and SO2 analyzers to determine if any of these gas products were formed over a nominal temperature range of 400-1000 °C. As earlier work had failed to find any carbon in the solid residues produced, it was initially assumed that CO itself ought not to be involved in any direct reaction with CaS.2 However, since elemental carbon might also be subsequently removed by direct reaction with CO2, at higher temperatures, it was decided to carry out an experiment with a CO gas stream. Finally, because conventional gas analyzers cannot detect COS, experiments were carried out in which the resulting gas streams were subjected to analysis using mass spectrometry. Subsequently, because of evidence of impurities in the first sample of CaS, a new sample of fresh CaS was obtained and further experiments were carried out on it. Experimental Section A tube furnace was used to carry out the tests. Typically, a small sample of CaS was placed in a ceramic boat, and the system was flushed with CO2 or CO for 20 min prior to heating the sample at a programmed rate. The particle size was fine (
